In the following, the invention will be explained in relation to a Solid Oxide Fuel Cell. The interconnect according to the invention can, however, also be used for other types of fuel cells such as Polymer Electrolyte Fuel cells (PEM) or a Direct Methanol Fuel Cell (DMFC). A Solid Oxide Fuel Cell (SOFC) comprises a solid electrolyte that enables the conduction of oxygen ions, a cathode where oxygen is reduced to oxygen ions and an anode where hydrogen is oxidised. The overall reaction in a SOFC is that hydrogen and oxygen electrochemically react to produce electricity, heat and water. In order to produce the required hydrogen, the anode normally possesses catalytic activity for the steam reforming of hydrocarbons, particularly natural gas, whereby hydrogen, carbon dioxide and carbon monoxide are generated. Steam reforming of methane, the main component of natural gas, can be described by the following equations:CH4+H2O→CO+3H2 CH4+CO2→2CO+2H2 CO+H2O→CO2+H2 
During operation, an oxidant such as air is supplied to the solid oxide fuel cell in the cathode region. Fuel such as hydrogen is supplied in the anode region of the fuel cell. Alternatively, a hydrocarbon fuel such as methane is supplied in the anode region, where it is converted to hydrogen and carbon oxides by the above reactions. Hydrogen passes through the porous anode and reacts at the anode/electrolyte interface with oxygen ions generated on the cathode side that have diffused through the electrolyte. Oxygen ions are created in the cathode side with an input of electrons from the external electrical circuit of the cell.
To increase voltage, several cell units are assembled to form a stack and are linked together by interconnects. Interconnects serve as a gas barrier to separate the anode (fuel) and cathode (air/oxygen) sides of adjacent cell units, and at the same time they enable current conduction between the adjacent cells, i.e. between an anode of one cell with a surplus of electrons and a cathode of a neighbouring cell which requires electrons for the reduction process. Further, interconnects are normally provided with a plurality of flow paths for the passage of fuel gas on one side of the interconnect and oxidant gas on the opposite side.
A solid oxide fuel cell (SOFC) stack is thus a sandwich composed of ceramic fuel cells and metal interconnects and spacers. These different materials are glued together at high temperature with glass seals to form a rigid structure. The use of such different materials makes it impossible to avoid some differences in thermal expansion coefficients (TEC). During operation, the stack can be subjected to high temperatures up to approximately 1000 degrees Celsius causing temperature gradients in the stack and thus different thermal expansion of the different components of the stack. The resulting thermal expansion may lead to a reduction in the electrical contact between the different layers in the stack. The thermal expansion may also lead to cracks and leakage in the gas seals between the different layers leading to poorer functioning of the stack and a reduced power output.
When the stack is cooled from the sealing temperature or the operation temperature, the mismatch in TEC values results in thermomechanic stresses and crack inducing energy. The potential energy which can be released when the endplate and the stack delaminates is approximately proportional to the thickness of the endplate and proportional to the square of the difference between the stack TEC and the endplate TEC. Hence, both the match of TEC values and the thickness of the endplates are crucial for the integrity of the cell stack. With thick endplates integral with the stack ends, the crack inducing energy will result in delamination of the stack and loss of integrity unless the stack is protected by a compression system.
A solution to this problem is disclosed in PCT/EP201/001938, where the thickness and the TEC values of the endplates are sought matched to the cell stack. However, thin endplates only partly solves the problems: the mismatch in TEC values will be a problem even with thin endplates if further components with different TEC values are applied to the cell stack. This is the case in state of the art cell stacks where the connection of process gas to the cell stack is done by means of thick metal plates. Therefore, there is a need for a process gas connection solution to fuel cell stacks, which solves the problem of TEC values of process gas connections, which do not match the cell stack TEC values.
EP0408104 discloses process gas supplies, which are connected to thin separator plates. However, the separator plates extend outside the area of the actual cell stack, therefore need a considerable extra amount of space, and excessively increases the total dimensions of the cell stack arrangement (FIG. 3). Further EP0408104 describes a spring loaded gas passage from the process gas supply to each cell in the stack to compensate for the shrinkage of the electrodes, a rather expensive solution as a cell stack comprises a large amount of cells.
WO02075893 also discloses a solution where process gas supplies are arranged outside the active area of the cell stack and at least some of the gas supplies are connected to rather thick plates.
A similar solution can be found in WO2008023879, where process gas supplies are connected to relative thick endplates.